In the present study, the patterns and mechanisms of ventricular activation of intraventricular conduction disturbances of ventricular arrhythmias will be investigated. Previous studies in man and animals have suggested that very similar patterns in the surface electrocardiogram may be produced by several different mechanisms which are identifiable by activation mapping. In this study, the sequence of epicardial, intramural and endocardial activation of the ventricles will be determined using techniques of cardiac catheterization and/or intraoperative mapping. In all studies, multiple recordings were made from the ventricular surface either endocardial or epicardial in addition to several ECG leads. From these data isochronic activation maps are constructed. Participation of various segments of ventricular specialized conduction system will be assessed by recordings made from these. These findings are correlated with the electrocardiogram and time normalized vectorcardiograms obtained before and after operation or before and after study. These techniques will be used to study right bundle branch block, left anterior hemiblock and left bundle branch block, right and left ventricular hypertrophy, ventricular tachycardia, and myocardial infarction. A more precise definition and understanding of the electrocardiogram will be obtained by these studies and will allow expansion of the clinical utility of these tests. In addition, new treatments for certain arrhythmias may be developed from this information.